The field of the invention is magnetic resonance imaging, and in particular local coils for using in magnetic resonance imaging of the head.
Magnetic resonance imaging (MRI) provides images, for example, of a human patient, by detecting faint signals from precessing hydrogen protons under the influence of a strong magnetic field and after a radio frequency excitation.
The quality of the image produced by MRI is strongly dependent on the strength of the received signal. For this reason, it is known to use radio frequency receiving coils placed in close proximity to the area being imaged. Such coils are called local coils.
A common local coil used for head or brain imaging is the so-called xe2x80x9cbird cagexe2x80x9d design providing a set of longitudinal conductors arrayed about the periphery of a cylindrical volume and terminated by conductor supports aligned with the bases of the cylindrical volume. The patient""s head fits through one conductor support into the enclosed volume. The coil may operate with the longitudinal conductors joined into a unitary resonant structure or with the longitudinal conductors isolated to provide a phased array of separate antennas.
Birdcage coils can be difficult to position on patients, often requiring one person to lift the patient""s head while the conductor support of the coil is positioned over the patient""s head. For this reason, split birdcage coils have been developed where the coil is separable along a horizontal plane bisecting the cylindrical volume. The lower portion of the coil forming a shallow trough may be pre-positioned on the table and the patient""s head located in this trough when the patient is positioned on the table. After the patient is in position, the upper portion of the coil may be fitted onto the lower portion to complete the coil.
A disadvantage of this design is that it requires two parts, one of which must be momentarily stored in another location while the patient is positioned. Generally, two hands are required to lift the top part of the coil into position.
In an alternative design, the patient""s head is positioned on a support held in cantilevered fashion so that the coil may slide around the support after the patient is in position. A drawback to this design is that the coil and support occupy a much greater space along the length of the table when they are separated, and in some cases, the coil may extend over the end of the table where it may catch on the bore of the magnet when the patient table is raised or lowered.
When a patient is intubated, for example, with an endotracheal tube, the tube may interfere with positioning of the patient in the coil or the coil about the patient.
The present invention provides a head coil that may be shortened along its longitudinal axis to allow easy placement of the patient onto a contained head rest. The coil is then lengthened over the patient for imaging. The resulting coil may be operated with one hand and does not require the storage or movement of separable parts. Because the coil is shortened, the end of the coil need not extend off the end of the table to interfere with the operation of the table and the magnet.
One conductor support of the coil may be equipped with a removable bridge so when the coil is lengthened, the conductor support may pass freely by an endotracheal tube or the like.
Specifically, the present invention provides an MRI head coil having a longitudinally extending head support sized to support the head of a supine patient. A first conductor support is positioned superior to the head support. A second conductor support having an opening for receiving the head support therethrough is mounted to move longitudinally with respect to the head support and the first conductor support. The second conductor support moves between a first position toward the first conductor support and a second position away from the first conductor support. A set of conductors extends longitudinally between the first and second conductor supports when the second conductor support is in the second position to define together with the first and second conductor supports, a volume substantially enclosing the patient""s head.
It is thus one object of the invention to produce a coil that may be shortened to allow access to the imaging volume for positioning and attending to the patient.
It is another object of the invention to provide a high degree of access to the patient prior to imaging without unduly increasing the footprint for supporting the head coil.
It is another object of the invention to provide a coil that allows easy access to the patient prior to imaging that may be operated with one hand and does not require the operator to leave the patient""s side in order to obtain another coil component.
The first conductor support may be mounted relative to the head support to also move longitudinally with respect to the head support.
Thus it is another object of the invention to provide greater access to the patient than can be obtained by shortening the coil alone.
The first and second conductor supports and head support can be mounted on a base sized to be received on a patient table.
Thus it is another object of the invention to provide a simple and stable mounting of the moveable components of the present design.
The second conductor support may be a ring surrounding the head support or an inverted U having arms flanking a stationary base wherein the base includes longitudinal conductors.
Thus it is another object of the invention to provide for a reduction in the need for collapsible conductors in the base portion which may remain uncompressed.
The longitudinal conductors may be connected to form a single resonant structure or phased array of separate antennas.
Thus it is another object of the invention to provide for a coil design and accommodates two principal types of head coils.
The longitudinal conductors may form a resonant circuit tuned to the NMR frequency when the second conductor support is in the second position and detuned when the second conductor is in the first position.
Thus it is another object of the invention to provide a coil that is automatically detuned and thus decoupled from RF fields when it is in the collapsed condition.
The second conductor support may include a removable bridge portion over a portion of the opening passing by the patient""s nose and mouth when the patient""s head is on the patient support and the second conductor support is moved from the first position to the second position.
Thus it is another object of the invention to provide a coil that does not interfere with endotracheal tubes and the like.
These particular objects and advantages may apply to only some embodiments falling within the claims and thus do not define the scope of the invention.